1. Field of the Invention
The present invention relates to a method of producing acids and alkalis using, as starting materials, ashes obtained by burning components contained in a black liquor that is obtained by digesting wood chips in the pulp and paper industry.
2. Prior Art
In a process of producing kraft pulps in the pulp and paper industry, digestion has been carried out in which a white liquor consisting of sodium sulfide and caustic soda is added to the wood chips to dissolve lignin. This digestion forms a waste liquor called black liquor which contains organic matters such as lignin and resin components as well as inorganic matters. The black liquor is concentrated and is burned in a recovery boiler, whereby inorganic matters are mostly reduced in the furnace, taken out from the lower part of the recovery boiler in the form of smelts (molten salts) consisting of sodium sulfide and sodium carbonate, and are recovered as a green liquor. Thereafter, the green liquor is mixed with slaked lime in a step of causticization, and the sodium carbonate contained in the green liquor is transformed into the caustic soda and is recovered as a white liquor which can be used again.
In burning the black liquor in the recovery boiler, on the other hand, fine powder of some inorganic matters is recovered as ashes by the economizer in the recovery boiler and by the electric precipitator. The ashes usually contain common salt to some extent. The ashes are all recovered and are often reused in the recovery boiler as make-ups of sulfur and sodium components. In case the common salt accumulates in the system to adversely affect the boiler, however, the ashes are neutralized and are discarded away.
However, the ashes obtained through burning in the recovery boiler are composed as chief components of those inorganic matters that can be reused as industrial starting material. Therefore, discarding the ashes away arouses problems not only in economy but also from the standpoint of polluting environment. It is therefore desired to recover the ashes as much as possible to reuse them.